The term "lignitic-type coal" as herein employed and as set forth in the subjoined claims, broadly encompasses a series of relatively low rank or low grade carbonaceous materials or coals including lignitic coals which encompasses lignite and brown coal, as well as subbituminous coals conventionally classified as rank A, B and C in order of their heating values. Lignitic coal comprises a carbonaceous low-grade coal which has not undergone a sufficient geological metamorphosis to convert it into a high-grade hard coal such as bituminous or anthracite. Lignitic coal broadly encompasses a range of such carbonaceous materials extending somewhere between peat and subbituminous coal, with brown coal being a form of lignite which is rather closely related to peat. Technically, lignite has been classified as those carbonaceous materials found in deposits similar to coal in which the carbon-hydrogen ratio varies from about 11.2:1 to 9.3:1. Subbituminous coals are of a higher degree of carbonification than lignitic coals and are ranked in accordance with a classification system as set out in United States Bureau of Mines, Bulletin No. 492, 1951, "Methods of Analyzing Coal and Coke", as Rank A having a moist heating value of 11,000 BTU or more, but less than 13,000 BTU; Rank B having a moist heating value of 9,500 BTU or more, but less than 11,000 BTU; and Rank C having a moist heating value of 8,300 BTU or more, but less than 9,500 BTU.
In the United States, vast deposits of lignitic coal are located in the north central states, principally in North and South Dakota and Wyoming, and to a lesser extent, in southern states, including Texas, while subbituminous coals are principally found in Washington, Wyoming and Colorado. These vast deposits represent a potential solution to the present energy crisis and fuel shortage. Unfortunately, lignitic-type coal as-mined, usually contains from about 20% up to about 40% moisture, of which at least a portion must first be removed to render it suitable as a fuel. A partial or complete drying of the moist lignitic-type coal results in a disintegration thereof into fine-sized particles and dust, posing not only problems due to spontaneous combustion, but also increasing the difficulty in handling it during shipment and firing into a furnace. The disintegration of the fuel when charged into furnaces causes portions thereof to fall through the furnace grates, as well as effecting a clogging thereof, detracting from the efficiency of the combustion operation and a substantial waste of the potential heating value thereof.
A variety of processes have heretofore been used or proposed for treating lignitic-type coal so as to render it more suitable as a solid fuel. Such prior art processes generally involve a partial drying of the lignitic-type coal in an as-mined condition to reduce its moisture content, and thereafter briquetting or agglomerating the material to render it more resistant to weathering and disintegration during shipment, storage and ultimate use. Typical of prior art processes for treating lignitic-type coals are those disclosed in U.S. Pat. Nos. 838,281; 1,205,007; 1,219,155; 1,386,472; 1,477,642; 1,508,617; 1,556,036; 1,577,902; 1,600,065; 1,698,345; 1,860,890; 1,871,862; 2,627,497; 2,903,400 and 3,723,079. The large investment in briquetting equipment, the large amount of labor required in the briquetting operation and the relatively high cost of the binding and/or coating agents employed has detracted from a more widespread commercial use of such processes and has impeded the utilization of the vast domestic deposits of lignitic-type coal to ease the present energy crisis.
The process of the present invention overcomes many of the problems and disadvantages associated with prior art techniques in which lignitic-type coals are upgraded in their physical structure and heating value, rendering them stable and resistant to disintegration during weathering, handling, storage and shipment, and suitable for use as a solid fuel alone or in admixture with high-grade coals, such as bituminous coal.